Brominated polystyrene is used as an additive to thermoplastics to impart flame retardant properties. In addition to thermal stability, it is necessary and desirable for these additives to impart essentially no color to the thermoplastic. The evolution of engineering thermoplastics has resulted in specialty polymers with much higher heat resistance and, as a result, a need to process these new materials at ever increasing temperatures. Because of higher and higher processing temperatures, the flame retardant additives used in these engineering thermoplastics must have a higher order of thermal stability and better color than that required in the past. Accordingly, this invention generally relates to a brominated polystyrene having improved color and thermal stability. More particularly, the invention relates to a process for the bromination of polystyrene which overcomes the limitations of current technology by use of an additive to suppress backbone halogenation.
Reports of the use of brominated polystyrene as a flame retardant additive in thermoplastics extend back more than twenty-five years. In 1980, Ferro Corporation, the Assignee of record herein, introduced brominated polystyrene as a commercial flame retardant additive under the trade name PyroChek(copyright) 68PB. The process for producing PyroChek(copyright) 68PB is described in U.S. Pat. No. 4,352,909. This product has become a leading flame retardant additive for use in reinforced engineering thermoplastics. More recently, Great Lakes Chemical has introduced a second brominated polystyrene product, PDBS-80, to the marketplace. This product also finds its primary application in engineering thermoplastics.
Thus, there are currently two different synthetic routes available for the commercial production of brominated polystyrenes. Each process has certain advantages and disadvantages which should be noted in order to fully understand the significance of the present invention.
The process used to produce PDBS-80, the commercial product offered by Great Lakes Chemical, is described in U.S. Pat. No. 5,369,202. It involves four chemical steps starting from styrene monomer. The first step involves the addition of HBr across the double bond of the styrene in order to protect it. In the second step, this intermediate is brominated on the ring using conventional technology. Usually an average of two bromines are introduced. The second intermediate is then reacted with strong inorganic base. This eliminates hydrogen bromine from the bromoethyl group of the second intermediate, reforming the double bond to produce brominated styrene monomer. After purification, this monomer is polymerized to form the brominated polystyrene product. The entire process may be represented as follows: 
This process has one significant advantage. It produces a brominated polystyrene which is essentially free of backbone halogen. This results in a product with very good thermal stability, good color, and good color stability. However, the process has two serious limitations which are major disadvantages when compared to the alternate process.
a. The process involves four distinct chemical reactions as well as several other unit operations. It is a complex process requiring a complex manufacturing facility with a high capital cost and the multiplicity of steps results in a long process. This process is inherently expensive.
b. Brominated styrene monomers are very reactive and difficult to handle. Ideally, a brominated aromatic flame retardant additive should have a high bromine content in order to have maximum efficiency and minimum cost. Thus, it would be preferable to produce and polymerize tribromostyrene monomer. However, this monomer is a highly reactive solid with low volatility. It is difficult to handle and polymerize and any residual monomer in the polymer would be difficult to remove. Consequently, this process tends to be limited to dibromostyrene as the maximum degree of bromination practical by this process. This limits the bromine content of the commercial brominated polystyrene (PDBS-80) to about 60%. Consequently, when used as a flame retardant additive, a relatively high use level is required to achieve flame retardance. This makes the product expensive to use. But of even greater concern to the user is the fact that high use levels cause deterioration of the important physical properties of the host resin. This result is frequently unacceptable to the user.
The process used by Ferro Corporation to produce its brominated polystyrene flame retardant additive. PyroChek(copyright) 68PB is described in the aforementioned U.S. Pat. No. 4,352,909. This process has many advantages over the process which involves the production and polymerization of brominated styrene monomer. Some of these include:
a. The process involves only a single chemical reaction, the bromination of commercially available polystyrene dissolved in a commercially available solvent using a commercially available brominating agent, bromine chloride. The process can be carried out in a simpler plant with a much lower capital cost. This process is inherently less expensive than the production of brominated polystyrene by the preparation and polymerization of brominated styrene monomer.
b. Because the process never involves the formation and handling of brominated styrene monomer, it does not have the limitations of the other process. It is possible to achieve tribromination and approach bromine contents of 70%. Since the brominating agent is less expensive than the polystyrene raw material, this actually reduces the cost of the product. Further, higher bromine contents result in lower use levels to achieve flame retardance. This reduces costs. But of even greater importance, reduced use levels result in better retention of physical properties of the host resin.
c. The process allows the use of a wide variety of polystyrenes and this, in turn, allows for the production of a variety of brominated polystyrenes. Further, general purpose, crystal polystyrene is produced in very large volumes in every part of the developed and developing world. This makes it readily available and inexpensive.
Notwithstanding the many advantages this process has over the process for making brominated polystyrene from monomer, a disadvantage exists which is beginning to limit the value and versatility of this product. In particular, while the process puts most of the bromine on the aromatic ring of the polystyrene, it also puts a small but significant amount of bromine and chlorine on the backbone. Typically, the amount of halogen, reported as HBr, on the backbone is 5000-6000 ppm, as measured by a test procedure described in detail hereinbelow. This backbone halogen is the direct cause of the limited thermal stability of brominated polystyrenes produced in this manner and is the direct cause of both its problems regarding initial color and color stability during thermal processing. Under the conditions of thermal processing, the backbone halogen of the current brominated polystyrenes produced in this manner may be released causing corrosion of processing equipment and degradation of the host resin. The formation of unsaturation in the backbone of the brominated polystyrene also leads to a loss of good color during processing. Since the technology trend in engineering thermoplastics is to higher and higher processing temperatures, the current brominated polystyrenes produced in this manner are becoming less acceptable in newer applications.
When brominated polystyrene is employed as a flame retardant additive in thermoplastics, its color is a property of primary importance to the manufacturer of the thermoplastic materials. The thermoplastic manufacturer desires to produce the thermoplastic articles in a wide range of colors. The more highly colored an additive, the more difficult it becomes to match (produce) a broad range of colors. The more lightly colored the additive, the easier it becomes to produce a wide range of colors. Therefore, in view of the needs of the manufacturer of thermoplastic parts, and in view of the inadequacy of prior art processes to produce a highly brominated polystyrene having the desired light color characteristics, a need exists for a highly brominated polystyrene with an improved light appearance as manufactured so that the end user can formulate a wide range of colors and thereby better meet the needs and demands of the marketplace.
It is therefore, an object of the present invention to provide a process for highly brominating polystyrenes which allows the direct bromination of polystyrene to produce a product with excellent thermal stability, excellent color, good color stability, and a minimum of backbone halogen.
It is another object of the present invention to provide a process which can be carried out in the existing facilities for the bromination of polystyrene without modification, without any additional capital investment, and with an absolute minimum of increase in raw material cost.
It is yet another object of the present invention to provide a process which utilizes an additive for the suppression of backbone halogenation, thereby allowing the operator to obtain highly brominated polystyrenes having improved thermal stability and color.
It is another object of the present invention to provide a highly brominated polystyrene having improved color and thermal stability and with less backbone halogen.
At least one or more of the foregoing objectives, together with the advantages thereof over existing prior art forms, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
In general, a process for the suppression of backbone halogenation during the bromination of polystyrenes comprises pretreating a solution of a polystyrene reactant comprising from about five to about 20 percent by weight of a polystyrene reactant, in a halogenated hydrocarbon solvent with at least about 0.1 percent by weight of an additive to suppress halogenation of the polystyrene backbone, prior to brominating the polystyrene in the presence of a catalyst; gradually adding a metal halide bromination catalyst to the solution, capable of effecting bromination of the ring without causing crosslinking of the polystyrene; adding to the solution from about 1 to 3.4 moles of a brominating agent, per mole of polystyrene reactant repeating units; reacting the polystyrene reactant with the brominating agent at a reaction temperature of from about xe2x88x9220xc2x0 C. to about 50xc2x0 C., and isolating the brominated polystyrene.
The present invention also provides a brominated polystyrene having a backbone halogen content of less than about 750 ppm and, a xcex94E color as low as about 5.